I went through this with my 15-89. I took the bobbin holder completely apart (yes, removed the little screw) and cleaned it thoroughly. I also took the upper tensioner completely apart and cleaned it thoroughly. I know my Mom kept the machine clean when she was using it regularly, but I found lint in the upper tensioner that looked like it had been there for 75 years (age of machine). Be sure to keep track of all the little parts and the order they go back together. After the thorough cleaning everything works wonderfully and I no longer have skipped stitches.